Woody Debris Management

Adapted from Ontario’s Stream Rehabilitation Manual

Description

Massachusetts’ rivers and streams contained numerous boulders, large fallen trees, logs and drift jams prior to settlement. In the first decades of colonization, pioneers cleared away debris and obstacles to allow for transportation, logging, mills, and the draining of floodplain forest to create pasture. In recent years, we have come to recognize the importance of large woody debris in river channels and the contribution it makes to channel function and fish habitat. This gives rise to the understanding that there are good log jams and then there are bad log jams.

Differentiating between the two requires knowledge of what instigated the problem from a watershed perspective, typical channel characteristics within the specific reach and the social and biological impacts at the local site level. The risk of severe flooding and erosion or fragmentation of aquatic habitats are concerns that are raised when instream woody debris causes channel obstructions.

Natural accumulations of large woody debris provide a variety of cover habitats for fish, aquatic insects, and wildlife. They also maintain channel stability by trapping sediment and organic matter while redirecting flow that scours pools and exposes larger substrates. The diameter, length, and the degree of being imbedded into the channel will indicate debris stability in the stream and whether it needs to be removed or repositioned. The width of the stream also dictates the typical orientation, with perpendicular logs common in small streams. Larger rivers typically force woody debris into a parallel direction with the current.

Large debris dams caused by changes in adjacent land use practices can have a devastating impact on fish migration, sediment transport, channel stability and increase the threat of flooding. When social values are at risk or some form of serious biological impact is identified, the obstructions should be removed in order to restore channel function and reduce possible harm. Otherwise, natural woody debris should be left alone because it’s important to the stream ecosystem. Debris that it is not natural (e.g. shopping carts, trash, etc.) should be removed.

Applicability

Recognizing the importance, identifying stable forms, and evaluating the function of large woody debris are fundamentals to determining which material should be removed or retained. For the most part, removal of debris obstructions will focus on instream material that has potential to increase ice jamming, flooding and erosion or present impassable barriers to fish migration. Debris removal, as a stream rehabilitation technique, is applied in cases where the watercourse or watershed has been significantly altered in terms of forest removal, catastrophic events, drastic changes in hydrology (increased stormwater runoff volume and frequency) or channelization. It is important to recognize early in the assessment that the actual removal should be the last resort.

Having knowledge of the channel slope, width, entrenchment, sinuosity and substrate combined with the length and diameter of the logs in the obstruction will provide the basis for determining whether the woody debris should be either removed from the floodplain or salvaged for creating log sills, deflectors or cover habitat. Channel width will indicate the size of woody material that can be considered stable in the stream and the manner in which it can be positioned in relation to the flow. Slope, entrenchment, sinuosity and substrate will define the channel forms that accommodate woody structures. In general, those streams and rivers are known for their ability to sustain large woody debris based on the stable nature of the channel and limited sediment supply.

Caution should be exercised in gravel, sand and silt-based streams where bank-placed large wood debris can aggravate erosion on the opposite bank. Generally, high gradient streams have an increased need for wood debris whereas low gradient streams are more likely to benefit from selective removal of woody debris. In addition, small streams might benefit from selected woody debris removal and larger streams benefit from adding wood debris.

Small streams (less than 24 feet wide) can support a larger amount of wood debris with a variety of diameters and lengths. In this case, most logs will orient perpendicularly or downstream to the flow. Those logs will create stable plunge pools, bank-side log jams, deflectors and instream cover when firmly anchored to the bank or bed. Hazardous debris jams are not a common concern in this situation. Larger streams and rivers (over 24 feet wide) typically transport debris smaller than 30 feet long to areas of accumulation such as log jams. Logs and trees greater than 30 feet in length will orient parallel to the high discharge flow and maintain a stable position if firmly anchored to the river bed or bank. Hazardous drift dams are more common in larger streams and rivers.

Woody Debris Removal Guidelines

In most cases, woody debris in a stream is not a concern. In some cases, the flow impediment, sediment accumulation and habitat fragmentation caused by the log jam might prompt the need for action. The guidelines below provide a step-by-step process for developing a removal plan based on consultation, assessment, restoring channel function, and creating cover habitat. Prior to embarking on a debris removal project, there are seven important steps to be followed toward establishing agreement in principle:

1. Ownership: Find out who owns the property at risk and ask (1) if they share a similar concern regarding the obstruction and (2) if they concur with the need for removal.

2. Consultation: Meet with your Conservation Commission to discuss the proposed removal and related permitting issues. Determine when the least amount of impact is likely to occur as a result of debris removal. Correct timing will help mitigate biological impacts. Ask what is expected in terms of sediment control and removal of woody debris.

3. Agreement: If consensus is reached with the approval agencies and land owner that a serious flooding, erosion or biological impact exists, request their assistance in designing an obstruction removal project.

4. Plan: Once the principle of obstruction removal has been established, you will then develop a detailed plan of action. As a general rule of thumb, the larger the channel, the more assessment and forethought required. You will need to document existing conditions related to typical channel characteristics and the nature of the obstruction.

5. Draw a plan-view sketch of the reach of stream indicating the location of the debris obstruction, thalweg, pool and riffles, access options. Take photographs.

6. Measure (1) channel widths in 30-foot increments for the reach and determine typical channel width, total length of reach and debris obstruction, (2) the drop in vertical head of the water level from one end of the debris jam to the other using a surveying scope or line level, (3) the average slope of reach with survey scope or line level, and (4) the length and diameter of a representative sample of wood debris.

7. Determine (1) if the debris is consolidated in sediment or not, (2) the nature of the sediment stored upstream of the obstruction (e.g. silt, sand, gravel, organic), (3) if the blockage provides grade control and if removal would accelerate erosion, (4) the predominate substrate for a typical section within the reach limits of the floodplain.

Once the above information has been documented, you will have the knowledge to proceed with developing the removal plan in consultation with the regulatory agencies and landowner. The measurements provide an understanding of the obstruction size and damming effect as well as the characteristics of the channel in a typical section. Determining the nature of accumulated sediment, degree of debris consolidation, typical substrate composition, and extent of floodplain will help you develop answers to questions such as:

What should the channel look like in the location of the obstruction?
How is this debris to be removed?
Is a partial removal sufficient to address the impact?
Who or what is going to remove it?
When is it to be removed?
How long will it take, given the resources available?
Is there a sufficient volume of stored sediment to warrant a phased removal over several weeks or months?
What is the closest access?
Where is the debris to go once removed?
Is the debris large enough to create instream cover habitat nearby or better suited for floodplain brush piles for wildlife?
What kind of habitat structures are suitable for this type of channel?

Documented explanations should be carefully thought out because they form the basis of your removal plan. Once you have achieved a level of confidence with these answers, the removal plan should illustrate the proposed channel form through the affected section, the points of access, the location for debris piles outside of the active floodplain, and the location and type of any prescribed sediment controls. You should also include the types and locations of rehabilitation structures such as bankside cabled log jams, sweeps, floating log cover, log sills, or deflectors.

Keep in mind that streams and rivers narrower than 24 feet will accommodate more wood debris than larger channels. In this case, most large wood salvaged from drift dams should be secured to the bed or back of the channel such that it is perpendicular to the flow or oriented downstream. Generally, expect to accommodate 18 to 20 pieces of large woody debris in a 300-foot section. Rivers wider than 24 feet usually deposit the debris in the floodplain or cause it to accumulate on the outside of bends, on top of large anchored logs and boulders. Large logs are anchored at one or both ends in parallel with the flow. In this situation, it is best to construct a log deflector, sweeper or bankside cabled log jam with the salvaged material.

For a detailed discussion of issues related to beaver dams and the removal of beaver dams and associated debris, see the following web link: http://www.mass.gov/dfwele/dfw/dfw_beaver_law.htm

Special Considerations

In some cases, the buildup of large woody debris can create a hazardous situation that requires immediate attention (e.g. if a large log becomes caught under a bridge, either threatening the stability of the bridge or the safety of the people using it). In a true emergency situation, some of the careful analysis described above will understandably need to be done in a more cursory manner, if at all.

A common motivation for many people to manage and remove large woody debris in streams is to enhance navigability. Paddling clubs and others seeking to keep waterways open for paddling should be educated about the functions and values of live and dead vegetation in and extending into the water. Woody debris that may be a nuisance to paddlers may be important structure for fish, perching birds or basking turtles. People seeking to clear channels for navigation should be encouraged to only clear the minimum amount necessary to enable their boats to squeeze through.

Cost and Maintenance

Under most circumstances, the expense of removing debris obstructions is limited to the cost of the crew and hand tools. Costs can escalate where heavy machinery is needed. For larger log jams, the phased approach might be required to reduce the amount of impact from sediment release. It is best to start at the downstream end and work upstream. Frequent monitoring of the area within the first year is needed to ensure that the removal has not aggravated erosion. Sediment scour will reveal buried debris that might require removal as well.

References

Ontario’s Stream Rehabilitation Manual, Ontario Ministry of Natural Resources. 2000.